Solar collectors are used to convert the sun's rays into heat energy, usually transferring this heat to a liquid or gas which may be transported to a point of use.
One type of conventional collector includes a flat plate to which is attached a metallic tubular coil. The flat plate is adapted to collect the sun's rays, convert these rays to heat energy and conduct this heat energy to the tubular coil. A fluid, either a gas or liquid, is circulated through the coil. The fluid is heated in the process. The heated fluid is then transported to some other location where heat energy is extracted normally for the ultimate purpose of heating a building structure or water, or cooling a building by means of absorbtion refrigeration.
In one form of collector, the flat plate has been fabricated of aluminum. Aluminum has the drawback of being subject to electrolysis with resultant corrosion. Unless careful precautions are taken, aluminum structures may deteriorate rapidly.
In another form, the flat plate and tubular coil are fabricated of copper which has been considered the best material for solar collectors. Copper provides almost twice the conductivity of aluminum. However, the cost of copper is extremely high. The total area required in many cases may result in the cost of the solar collector being prohibitive.
Copper collectors usually consist of a hairpin or serpentine (or other) flat copper tubing attached to a copper plate. The coil may be mechanically held against the plate by means of a clamp. More often, the coil is soft soldered to the plate by means of a lead-tin solder. In either case, the method of attaching the coil to the plate is considered to be a bottleneck in the transfer of heat from the plate to the coil. A mechanical joint does not provide the best heat transfer connection. Additionally, corrosion, which generally occurs after a period of use, results in the mechanical joint being even less effective. The "soft solder" connection also often used is a bottleneck because the conductivity of lead-tin solder is only one-eighth that of copper and "soft solder" is also susceptible to corrosion.
Steel has not been extensively used in the past for solar collector flat plates or tubular coils because of the generally low heat conductivity properties of steel. In accordance with the present invention, a flat steel plate and steel tubular coil are used. The steel coil is hydrogen copper brazed to the flat steel plate. This process results in a fillet on each side of the tube and a flat pool of copper between the fillets joining the steel tube and steel plate. The result is that heat is conducted from the steel plate to the steel tube at approximately ten times the rate that it would be in an all copper collector where the conduction between plate and tube is made through lead-tin solder or a mechanical connection. While it is true that copper is a much better conductor than steel, the relatively low cost of steel permits heavier gages of steel to be used thereby offsetting some of this advantage. The steel tubes may also be placed closer together thereby further reducing the advantage of the superior heat conductivity enjoyed by copper. By this means, the performance difference between a collector employing copper and a collector employing steel as the collecting device is practically eliminated.
A further advantage of the present invention is that hydrogen copper brazing is a more economical way to attach a coil to a plate than is soft soldering. Copper plates and tubing could not be hydrogen copper brazed because in this process, the structure is passed through a hydrogen atmosphere control furnace at approximately 2100.degree. Fahrenheit. At this temperature, copper melts which in the nature of things is desired in connection with brazing steel parts together but would be impossible for use in connection with brazing copper parts together. Hydrogen brazing is also desirable because it is a clean process minimizing the necessity to clean the finished parts.
In essence, the hydrogen copper brazed steel collector can approach the efficiency of a copper collector and possibly in some cases exceed such efficiency. However, the hydrogen copper brazed steel collector is only a fraction of the cost of a copper tubing and copper plate collector.